The Internet currently supports worldwide communication among computers using various standard protocols. One of these protocols, the Internet Protocol (IP), assigns a unique address to each computer known as the IP address. IP is currently available in two versions: IPv4 with 32 bit addresses, and IPv6 with 128 bit addresses. IPv4 is the most common version in use today.
Growth of the Internet has used all of the available 32 bit addresses in IPv4. One result of the limited number of addresses is that most organizations now use one of the three private address spaces defined by IPv4. These private IP addresses cannot be used on the public Internet. Gateway routers manage the interface between the private intranet and the public Internet. Gateway routers provide various functions to hide or mask the private internal IP when communication outside the private network is desired.
One common method used by gateway routers in commercial environments is the creation of a Virtual Private Network (VPN) to connect external users to the internal private network. A VPN provides an envelope or wrapper protocol to hide the internal IP addresses and data while the packet is routed across the public Internet to the client workstation.
The VPN extends the internal private network by assigning an internal private IP address to the client workstation as the client workstation connects to the VPN gateway. The VPN creates a network or VPN tunnel that connects the applications on the client workstation to the internal private network behind the VPN gateway (or owner gateway). The client workstation's local private network and the public Internet are hidden from applications on the client workstation by the VPN tunnel. As a result, in current versions of VPN, the client workstation can only connect to one VPN at a time. If a client workstation were able to connect to more than one VPN then, since the internal private address realms for each VPN are not guaranteed to be unique, packets could not be reliably routed to the desired destinations.